Method for reporting channel information in multiple antenna system

ABSTRACT

There is provided a method of reporting downlink channel information to a base station in a multiple antenna system. The method includes reporting a single rank for overall subband, the overall subband comprising a plurality of subbands and reporting a CQI for the single rank for at least one subband. Radio resources required for reporting channel information can be reduced and signaling overheads can be minimized.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority of U.S. Provisionalapplication Ser. No. 60/863,111 filed on Oct. 26, 2006 and Korean PatentApplication No. 10-2006-0135960 filed on Dec. 28, 2006, which areincorporated by reference in their entirety herein.

BACKGROUND

1. Technical Field

The present invention relates to wireless communication, and morespecifically, to a method of reporting downlink channel information in amultiple antenna system.

2. Related Art

Owing to generalization of information communication services, advent ofa variety of multimedia services, appearance of high quality services,and the like, the demand for communication services are rapidlyincreased. Researches on a variety of wireless communication techniquesare in progress in various fields to satisfy such demand.

A multiple-input multiple-output (MIMO) technique capable ofsimultaneously transmitting multiple spatial streams is required toobtain high spectral efficiency. The MIMO technique employs a multipletransmit antenna and one or more receiving antennas.

MIMO channels provided by a multiple antenna can be decomposed intomultiple independent channels. If the number of transmit antennas is Ntand the number of receiving antennas is Nr, the number of independentchannels Ni is Ni≦min{Nt, Nr}. Each of the independent channels can bereferred to as a spatial layer.

A rank is the number of non-zero eigenvalues of a MIMO channel matrix,which can be defined as the number of spatial streams that can bemultiplexed. The rank is the same as the number of independent channels.If the rank is one, one stream can be transmitted on one spatial layer,and if the rank is two, two independent streams can be simultaneouslytransmitted on two spatial layers. If the rank is K, K independentstreams having different rates can be transmitted on each spatial layer.

For a 4×4 MIMO system, maximum four ranks (four MIMO layers) arepossible. However, transmission using a maximum rank is not alwaysdesirable. A MIMO channel can limit a rank used for transmission.Although high rank transmission is superior to low rank transmission inthe aspect of a rate, the low rank transmission is desirable for a poorchannel condition.

In order to obtain gain for multiple antennas, it is needed to design aMIMO system that utilizes channel dependent feedback of a user equipmentto tune downlink transmission scheme. For this purpose, it is requiredthat the user equipment feeds back channel information.

If the user equipment reports channel information for every resourceblock, the best flexibility can be obtained. However, if the channelinformation for every resource block is reported, high signalingoverhead may be caused.

There is a need for a method which can reduce signaling overhead due tochannel information in the MIMO system.

SUMMARY

An object of the invention is to provide a method for reporting channelinformation to reduce signaling overhead in a multiple antenna system.

In one aspect, there is provided a method of reporting downlink channelinformation to a base station in a multiple antenna system. The methodincludes reporting a single rank for overall subband, the overallsubband comprising a plurality of subbands and reporting a CQI for thesingle rank for at least one subband.

In another aspect, there is provided a method of reporting downlinkchannel information to a base station in a multiple antenna system. Themethod includes selecting a single rank for overall subband, the overallsubband comprising a plurality of subbands, reporting the single rankand reporting a CQI for the single rank for each subband.

In still another aspect, there is provided a method for transmittingdownlink data in a multiple antenna system. The method includesreceiving a single rank for overall subband, receiving a CQI for thesingle rank, transmitting a rank determined using the single rankthrough a downlink control channel, allocating at least one subbandusing the rank and the CQI and transmitting the downlink data throughthe allocated subband.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing a transmitter according to anembodiment of the invention.

FIG. 2 is a block diagram showing a receiver according to an embodimentof the invention.

FIG. 3 is a view showing the resource configuration of a system based onOFDMA.

FIG. 4 is a flowchart illustrating a method of reporting channelinformation according to an embodiment of the present invention.

FIG. 5 is an exemplary view illustrating a method of reporting channelinformation according to an embodiment of the present invention.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

The technique described below can be used in a variety of communicationsystems including a code division multiple access (CDMA) system, awideband CDMA (WCDMA) system, a frequency division multiple access(FDMA) system, an orthogonal frequency division multiplexing(OFDM)/orthogonal frequency division multiple access (OFDMA) system, andthe like. OFDM is a multiple carrier modulation technique forefficiently dividing an overall system bandwidth into a plurality oforthogonal subbands. A subband can be referred to as a tone,subcarriers, a subchannel or the like.

The communication system can be a multiple-input multiple-output (MIMO)system or a multiple-input single-output (MISO) system. The MIMO systemuses a plurality of transmit antennas and a plurality of receivingantennas. The MISO system uses a plurality of transmit antennas and asingle receiving antenna.

A base station (hereinafter, referred to as BS) is a fixed stationcommunicating with a user equipment, which can be referred to as anotherterminology, such as a node-B, a base transceiver system (BTS), a accesspoint or the like. The user equipment (hereinafter, referred to as UE)can be fixed or mobile and can be referred to as another terminology,such as a mobile station (MS), a user terminal (UT), a subscriberstation (SS), a wireless device or the like.

A downlink means a communication from the BS to the UE, and an uplinkmeans a communication from the UE to the BS. In the downlink, atransmitter can be a part of the BS, and a receiver can be a part of theUE. In the uplink, the transmitter can be a part of the UE, and thereceiver can be a part of the BS.

FIG. 1 is a block diagram showing a transmitter according to anembodiment of the invention.

Referring to FIG. 1, a transmitter 100 includes a scheduler 110, channelencoders 120-1 to 120-K, mappers 130-1 to 130-K, MIMO processors 140-1to 140-K and a multiplexer 150. The transmitter 100 also includes Nt(Nt>1) transmit antennas 190-1 to 190-Nt.

The scheduler 110 receives data from N users and outputs K streams to betransmitted at one time. The scheduler 110 selects modulation and codingscheme (MCS) such as a code rate and modulation scheme and outputs theselected MCS to the channel encoders 120-1 to 120-K, the mappers 130-1to 130-K. The scheduler 110 selects MIMO scheme and outputs the selectedMIMO scheme to the MIMO processors 140-1 to 140-K.

Each of the channel encoders 120-1 to 120-K encodes input streams in apredetermined coding scheme and forms coded data. Each of the mappers130-1 to 130-K maps the coded data to a data symbol on signalconstellation. Any kind of modulation scheme can be used, includingm-Phase Shift Keying (m-PSK) and m-Quadrature Amplitude Modulation(m-QAM). For example, the m-PSK can be binary-PSK (BPSK), quadrature-PSK(QPSK), or 8-PSK, and the m-QAM can be 16-QAM, 64-QAM, or 256-QAM.

Each of the MIMO processors 140-1 to 140-K processes the data symbol inthe MIMO scheme in accordance with the multiple transmit antennas 190-1to 190-Nt. For example, the MIMO processors 140-1 to 140-K can performcodebook-based preceding.

The multiplexer 150 allocates an input symbol to an appropriate asub-carrier and multiplexes input symbols for multiple users. An OFDMmodulator 160 performs OFDM modulation on the input symbols and outputsan OFDM symbol. The OFDM modulator 160 can perform inverse fast Fouriertransform (IFFT) on the input symbols and additionally insert a cyclicprefix (CP) after performing the IFFT. The OFDM symbol is transmittedthrough each of the transmit antennas 190-1 to 190-Nt.

The transmitter 100 can operate in two modes. The one is a singlecodeword mode and the other is a multiple codeword mode. In the singlecodeword mode, signals transmitted through MIMO channels have the samedata rate. In the multiple codeword mode, data transmitted through theMIMO channels are independently encoded, so that transmission signalsmay have different data rates.

FIG. 2 is a block diagram showing a receiver according to an embodimentof the invention.

Referring to FIG. 2, a receiver 200 includes an OFDM demodulator 210, ademapper 240, a channel decoder 250, and a controller 260.

The OFDM demodulator 210 performs fast Fourier transform (FFT) onsignals received from receiving antennas 290-1 to 290-Nr. A channelestimator 220 estimates a channel, and a MIMO post-processor 230performs a post-process corresponding to the MIMO processors 140-1 to140-K. The demapper 240 demaps input symbols into coded data, and thechannel decoder 250 decodes the coded data and restores original data.The controller 260 creates appropriate feedback information and feedsback the created feedback information to the transmitter 100 through theestimated channel or the like.

FIG. 3 is a view showing the resource configuration of a system based onOFDMA.

Referring to FIG. 3, a system bandwidth is divided into a plurality ofsubbands. A subband is a unit of frequency resources allocated to eachUE. The subband also can be called as a resource block or a subchannel.Each UE can be allocated with at least one subband.

It is assumed that the system bandwidth is divided into 512 subcarriers,i.e., the size of FFT is 512. A subband includes twelve subcarrier andtotal number of the subbands is 25 (L=25). Guard bands are provided atboth ends of the system bandwidth.

A BS should know downlink channel information to select K UEs from N UEs(K<N), where K and N are integer. The BS allocates at least one subbandto a user using the channel information reported from the UE. Thedownlink channel information may include a channel quality indication(CQI), a rank and a preceding matrix index (PMI). Base on the channelinformation, the BS allocates radio resources to each UE based on anappropriate criterion.

Since the minimum transmission unit that can be allocated to each UE isthe subband, all of channel information needs to be calculated andtransmitted in correspondence with the subband. If the number of UEs ina sector or a cell is small, a plurality of subbands can be allocated toa UE.

If M subbands are allocated to a UE, the BS should inform the UE ofinformation on resource allocation and selected MCS and MIMO schemethrough a downlink channel. When L subbands are selected for the UE, theBS should transmit information for L selected subbands on a downlinkcontrol channel to the UE. This causes heavy traffic load for downlinkcontrol signals.

An appropriate MCS and a rank may be different in each subband. If onlythe MCS is different and M subbands are allocated to a UE, an averageCQI of the M subbands can be calculated as shown

$\begin{matrix}{{CQI} = {{\exp \left( {\frac{1}{M}{\sum\limits_{i = 1}^{M}\; {\log \left( {1 + {CQI}_{i}} \right)}}} \right)} - 1}} & \left\lbrack {{Mathematical}\mspace{14mu} {expression}\mspace{14mu} 1} \right\rbrack\end{matrix}$

where a CQI_(i) is a CQI of the i-th subband.

When a rank of each subband is different, an average CQI cannot becalculated using above equation. If the UE transmits CQIs for every rankas feedback information, the BS can transmit data through the best rank.However, the amount of feedback information is increased.

FIG. 4 is a flowchart illustrating a method of reporting channelinformation according to an embodiment of the present invention.

Referring to FIG. 4, a signal-to-interference plus noise ratio (SINR) ofa subband is calculated for each rank S220.

A single rank is determined for overall subbands based on a specificcriterion S230.

A UE selects the single rank for overall subbands based on the specificcriterion and reports the single rank and a CQI corresponding to thesingle rank S230. If codebook-based preceding is used, the UE can reporta PMI together with the single rank and the CQI.

The UE can reduce feedback overheads by reporting only the single rankselected for overall subbands to the BS. The BS receives the single rankand the CQI corresponding to the single rank. And the BS allocates atleast one subband to the UE for downlink data.

The UE can calculate a metric for each rank in order to determine thesingle rank. The metric can be calculated using SINR calculated for eachsubband and each rank.

In one embodiment, a throughput or a capacity of each rank is calculatedas a metric for determining a single rank. A rank having the largestvalue is selected as the single rank.

In the case of a single codeword mode, a single rank can be determinedas shown

max_(r)(max_(b)(ƒ(SIN R_(r,b))))  [Mathematical expression 2]

where b is the index of a subband, r is the index of a rank and SINR_(r,b) is SIN R of the r-th rank and b-th subband. f( ) is a functionof SINR, representing a capacity or a throughput, and its value becomesa metric.

By comparing with values of the metric for every rank, a rank having thelargest value of the metric is selected as the single rank.

For example, it is assumed that there is a system having eight subbandsand two ranks, i.e., rank 1 and rank 2. Metrics are as shown in Table 1.

TABLE 1 Rank 1 Rank 2 Subband 1 1.0 1.8 Subband 2 1.2 2.4 Subband 3 0.41.3 Subband 4 1.1 0.8 Subband 5 2.5 1.4 Subband 6 1.8 1.3 Subband 7 0.90.7 Subband 8 0.6 1.0

The largest value of the metric of rank 1 is 2.5 of subband 5, and thelargest value of the metric of rank 2 is 2.4 of subband 2. Accordingly,a single rank is determined as rank 1, and a corresponding CQI, acodebook index or a PMI is fed back.

In the case of a multiple codeword mode, a single rank can be determinedas shown

$\begin{matrix}\left. {\max_{r}\left( {\max_{b}\left( {\sum\limits_{i = 1}^{Cr}\; {f\left( {SINR}_{r,b,i} \right)}} \right)} \right)} \right) & \left\lbrack {{Mathematical}\mspace{14mu} {expression}\mspace{14mu} 3} \right\rbrack\end{matrix}$

where C_(r) is the number of codewords of rank r and SIN R_(r,b,i) isSIN R of the r-th rank, b-th subband, and i-th codeword. By comparingthe sum of C_(r) metrics for every ranks, a rank having the largest sumis selected as a single rank.

For example, it is assumed that there is a system having eight subbandsand two ranks, i.e., rank 1 and rank 2. Metrics are as shown in Table 2.

TABLE 2 Rank 1 Rank 2 Subband 1 1.0 0.8 1.0 Subband 2 1.2 1.0 1.4Subband 3 0.4 0.8 0.5 Subband 4 1.1 0.6 0.2 Subband 5 2.5 0.7 0.7Subband 6 1.8 0.6 0.7 Subband 7 0.9 0.4 0.3 Subband 8 0.6 0.5 0.5

C₁ of rank 1 is one, and C₂ of rank 2 is two. The largest sum of themetric of rank 1 is 2.5 of subband 5, and the largest sum of the metricof rank 2 is 2.4 of subband 2. Accordingly, a single rank is determinedas rank 1, and a corresponding CQI, a codebook index or a PMI is fedback.

In another embodiment, a throughput sum (or a capacity sum) of subbandshaving J best throughputs (or capacities) is calculated for each rank asa criterion for determining a single rank. A rank having the largestthroughput sum (or capacity sum) can be selected as the single rank. Jis a parameter determined depending on the number of subbands, afeedback method, a MIMO scheme and the like. J can be a value previouslystored in a UE's memory. Or, J can be a value previously known to both aBS and a UE or can be transmitted by the BS to the UE.

In the case of a single codeword mode, a single rank can be determinedas shown

$\begin{matrix}{\max_{r}\left( {\sum\limits_{h = 1}^{J}\; {{order}\left( {f\left( {SINR}_{r,b} \right)} \right)}} \right)} & \left\lbrack {{Mathematical}\mspace{14mu} {expression}\mspace{14mu} 4} \right\rbrack\end{matrix}$

where order( ) is a function for sorting internal values in descendingorder.

Metrics are calculated for each rank and sorted in descending order foreach subband and best J subbands of each rank are summed. By comparingwith metric sums of every ranks, a rank having the largest metric sum isselected as the single rank.

For example, it is assumed that there is a system having eight subbandsand two ranks, i.e., rank 1 and rank 2. Metrics are as shown in Table 3.

TABLE 3 Rank 1 Rank 2 Subband 1 1.0 1.8 Subband 2 1.2 2.4 Subband 3 0.41.3 Subband 4 1.1 0.8 Subband 5 2.5 1.4 Subband 6 1.8 1.3 Subband 7 0.90.7 Subband 8 0.6 1.0

It is assumed that J is four. In Table 3, the metric sum of best fourmetrics (subband 2, 4, 5, and 6) of rank 1 is 6.6, and the metric sum ofbest four metrics (subband 1, 2, 3, and 5) of rank 2 is 6.9.Accordingly, the single rank is determined as rank 2, and acorresponding CQI, a codebook index or a PMI is fed back.

In the case of a multiple codeword mode, a single rank can be determinedas shown

$\begin{matrix}{\max_{r}\left( {\sum\limits_{h = 1}^{J}\; {{order}\left( {\sum\limits_{i = 1}^{Cr}\; {f\left( {SINR}_{r,b,i} \right)}} \right)}} \right)} & \left\lbrack {{Mathematical}\mspace{14mu} {expression}\mspace{14mu} 5} \right\rbrack\end{matrix}$

where C_(r) is the number of codewords of rank r.

A sum of C_(r) metrics is calculated for every subbands of each rank andsorted in descending order in each subband. Best J metric sums of eachrank are summed. By comparing with metric sums of every ranks, a rankhaving the largest metric sum is selected as the single rank.

For example, it is assumed that there is a system having eight subbandsand two ranks, i.e., rank 1 and rank 2. Metrics are as shown in Table 4.

TABLE 4 Rank 1 Rank 2 Subband 1 1.0 0.8 1.0 Subband 2 1.2 1.0 1.4Subband 3 0.4 0.8 0.5 Subband 4 1.1 0.6 0.2 Subband 5 2.5 0.7 0.7Subband 6 1.8 0.6 0.7 Subband 7 0.9 0.4 0.3 Subband 8 0.6 0.5 0.5

C₁ of rank 1 is one, and C₂ of rank 2 is two. It is assumed that J isfour. In Table 4, the sum of largest four metric (subband 2, 4, 5, and6) of rank 1 is 6.6, and the sum of largest four metric sums (subband 1,2, 3, and 5) of rank 2 is 6.9. Accordingly, the single rank isdetermined as rank 2.

Even in the case of feeding back all CQIs of every subbands, feedingback CQIs of some subbands, feeding back CQIs using discrete cosinetransform (DCT) or the like, a single rank for overall subbands can bedetermined. A UE obtains a CQI of each subband for the single rank andreports the single rank and the CQI to the BS. The UE can report CQIsfor every subbands or CQIs for some subbands. When the number of overallsubbands is twelve, the UE obtains a CQI for each of the twelve subbandsand can report the twelve CQIs. Alternatively, the UE can select threesubbands having best CQIs out of the twelve subbands and report thethree CQIs. For the other nine subbands, an average CQI of the ninesubbands can be reported.

Hereinafter, A method of determining a single rank when CQIs of best Msubbands among L subbands (M<L) are reported and an average CQI isreported for the other subbands is described. A value M is a valuepreviously known to both a BS and a UE or can be transmitted by the BSto the UE.

When a throughput (or a capacity) of each rank is calculated as acriterion for determining a single rank, and a rank having the largestvalue is selected as the single rank, the rank can be determined usingmathematical expression 2 in a single codeword mode or mathematicalexpression 3 in a multiple codeword mode.

When a sum of J throughputs (or capacities) of each rank is taken as acriterion for determining a single rank, if M is larger than J, thesingle rank can be determined using mathematical expression 4 in asingle codeword mode and mathematical expression 5 in a multiplecodeword mode.

If M is smaller than J, the single rank in a single codeword mode can bedetermined as shown

$\begin{matrix}{\max_{r}\left( {\sum\limits_{b = 1}^{M}\; {{order}\left( {f\left( {SINR}_{r,b} \right)} \right)}} \right)} & \left\lbrack {{Mathematical}\mspace{14mu} {expression}\mspace{14mu} 6} \right\rbrack\end{matrix}$

For example, it is assumed that there is a system having eight subbandsand two ranks, i.e., rank 1 and rank 2. Metrics are as shown in Table 5.

TABLE 5 Rank 1 Rank 2 Subband 1 1.0 1.8 Subband 2 1.2 2.4 Subband 3 0.41.3 Subband 4 1.1 0.8 Subband 5 2.5 1.4 Subband 6 1.8 1.3 Subband 7 0.90.7 Subband 8 0.6 1.0

It is assumed that J is four and M is two. Since M is smaller than J,the rank is determined based on a sum of M metrics. That is, the sum oftwo best metrics (subbands 5 and 6) of rank 1 is 4.3, and the sum of twobest metrics (subbands 1 and 2) of rank 2 is 4.2. Accordingly, thesingle rank is determined as rank 1.

If M is smaller than J, a single rank in a multiple codeword mode can bedetermined as shown

$\begin{matrix}{\max_{r}\left( {\sum\limits_{h = 1}^{M}\; {{order}\left( {\sum\limits_{i = 1}^{Cr}\; {f\left( {SINR}_{r,b,i} \right)}} \right)}} \right)} & \left\lbrack {{Mathematical}\mspace{14mu} {expression}\mspace{14mu} 7} \right\rbrack\end{matrix}$

For example, it is assumed that there is a system having eight subbandsand two ranks, i.e., rank 1 and rank 2. Metrics are as shown in Table 6.

TABLE 6 Rank 1 Rank 2 Subband 1 1.0 0.8 1.0 Subband 2 1.2 1.0 1.4Subband 3 0.4 0.8 0.5 Subband 4 1.1 0.6 0.2 Subband 5 2.5 0.7 0.7Subband 6 1.8 0.6 0.7 Subband 7 0.9 0.4 0.3 Subband 8 0.6 0.5 0.5

It is assumed that J is tour and M is two. Since M is smaller than J,the single rank is determined based on a sum of M metrics. The sum oftwo largest metric sums of rank 1 is 4.3 and the sum of two largestmetric sums of rank 2 is 4.2. Accordingly, the single rank is determinedas rank 2.

Hereinafter, specific examples are described to show advantages of theproposed method.

It is assumed that the mode is a multiple codeword mode, and C₁ of rank1 is one, and C₂ of rank 2 is two. As MIMO scheme, cyclic delaydiversity (CDD) is used for rank 1, and generalized CDD is used for rank2. A receiver uses successive interference cancellation (SIC) as areceiving technique of rank 2. It is also assumed that the FFT size is512, one subband includes 36 subcarriers and 10 OFDM symbols and thereare eight subbands in total. 10 UEs are in a sector and scheduled byconventional proportional fair algorithm.

It is assumed that a CQI for one rank per subband is fed back. The rankof each subband can be differed.

First Example

One subband is allocated to a UE and the UE feeds back a rank and acorresponding CQI for each subband. The BS informs the UE of informationon resources allocated to each subband (MCS, MIMO scheme and the like).

CQIs measured by the UE for each rank are shown in Table 7 (the unit isdecibel).

TABLE 7 Rank 1 Rank 2 Subband 1 2.35 0.88 2.35 Subband 2 3.66 2.35 4.85Subband 3 −3.08 0.88 −1.88 Subband 4 3.02 −0.85 −6.55 Subband 5 10.490.06 0.06 Subband 6 7.03 −0.85 0.06 Subband 7 1.64 −3.08 −4.56 Subband 8−0.85 −1.88 −1.88

If capacity f(SINR)=log(1+CQI) is used as a criterion, metricscalculated using the measured CQIs are shown in Table 8.

TABLE 8 Rank 1 Rank 2 Subband 1 1.4 1.1 1.4 Subband 2 1.7 1.5 2.0Subband 3 0.9 1.1 0.7 Subband 4 1.6 0.8 0.3 Subband 5 3.6 1.0 1.0Subband 6 2.6 0.8 1.0 Subband 7 1.3 0.6 0.4 Subband 8 0.8 0.7 0.7

In the case of subbands 1, 2, 3 or 8, since the sum of metrics of rank 2is larger than the metric of rank 1, rank 2 is selected. In the case ofsubbands 4, 5, 6, or 7, since the sum of metrics of rank 2 is smallerthan the metric of rank 1, rank 1 is selected. Since feedbackinformation is rank information of each subband and a corresponding CQI,the UE feeds back channel information as shown in Table 9.

TABLE 9 Rank information CQI Subband 1 2 0.88 2.35 Subband 2 2 2.35 4.85Subband 3 2 0.88 −1.88 Subband 4 1 3.02 Subband 5 1 10.49 Subband 6 17.03 Subband 7 1 1.64 Subband 8 2 −1.88 −1.88

Second Example

A plurality of subbands is allocated to a UE and the UE feeds back arank of each subband and a corresponding CQI. The BS informs the UE ofinformation on resources allocated to each subband (MCS, MIMO scheme andthe like).

For clarity, CQIs and metrics of Tables 7 and 8 are used. Each UE feedsback the feedback information shown in Table 10.

TABLE 10 Rank information CQI Subband 1 2 0.88 2.35 Subband 2 2 2.354.85 Subband 3 2 0.88 −1.88 Subband 4 1 3.02 Subband 5 1 10.49 Subband 61 7.03 Subband 7 1 1.64 Subband 8 2 −1.88 −1.88

This example is the same as the first example in that the UE reports allof the channel information of each subband. However, they are differentin that only one subband is allocated to the UE in the first example,whereas a plurality of subbands is allocated to the UE in this example.Accordingly, downlink control information is reduced compared with thefirst example.

Third Example

A plurality of subbands is allocated to a UE and the UE selects a singlerank which is the rank having the largest value of the metric.

For clarity, CQIs and metrics of Tables 7 and 8 are used. Comparing ametric of rank 1 with a sum of metrics of rank 2 from subband 1 tosubband 8, the metric of 3.6 of subband 5 is the largest (refer tomathematical expression 3). Accordingly, rank 1 is selected as thesingle rank. A corresponding CQI of each subband is fed back. Table 11shows reported channel information.

TABLE 11 Rank information = Rank 1 CQI Subband 1 2.35 Subband 2 3.66Subband 3 −3.08 Subband 4 3.02 Subband 5 10.49 Subband 6 7.03 Subband 71.64 Subband 8 −0.85

CQI values can be transmitted as they are, or a difference value from aprevious value can be transmitted.

The UE reports the single rank and CQI of subbands for the single rank.Compared with the first example and the second example, the amount ofradio resources for transmitting channel information is reduced.

Fourth Example

A plurality of subbands is allocated to a UE and the UE selects a singlerank based on a sum of J best subbands.

For clarity, CQIs and metrics of Tables 7 and 8 are used. It is assumedthat J is four. Best subbands of rank 1 are in order of subbands 5, 6,2, and 4, and the sum of their metrics is 9.6. Best subbands of rank 2are in order of subbands 2, 1, 5, and 3, and the sum of their metrics is9.8. Accordingly, rank 2 is selected as the single rank (refer tomathematical expression 5) and a corresponding CQI of each subband isfed back. Table 12 shows feedback information.

TABLE 12 Rank information = Rank 2 CQI Subband 1 0.88 2.35 Subband 22.35 4.85 Subband 3 0.88 −1.88 Subband 4 −0.85 −6.55 Subband 5 0.06 0.06Subband 6 −0.85 0.06 Subband 7 −3.08 −4.56 Subband 8 −1.88 −1.88

The UE selects the single rank based on a sum of J best subbands andreports a CQI of the single rank. A plurality of subbands is allocatedto each UE. Compared with the first example and second example, theamount of radio resources for transmitting channel information isreduced.

Table 13 shows spectral efficiencies of the examples described above.

TABLE 13 First Second Third Fourth example example example exampleSpectral 1.78 2.03 1.91 1.93 efficiency (bps/Hz/sector)

Although the third and fourth examples nave a difference of about 6% inperformance compared with the second example, the amount of reportedchannel information is small in the third and fourth examples. Theamount of information on resource allocation informed by the BS to theUE is also small. The amount of the reported channel information isabout (the number of subbands)×log₂(# of available ranks) bits in thefirst and second examples and about 1×log₂(# of available rinks) bits inthe third and fourth examples.

Consequently, although a UE reports a single rank and a CQI for thesingle rank, the performance degradation does not occur. Signalingoverhead can be minimized.

FIG. 5 is an exemplary view illustrating a method of reporting channelinformation according to an embodiment of the present invention.

Referring to FIG. 5, the system bandwidth is divided into a plurality ofprimary bands. A primary band has a bandwidth narrower than the systembandwidth and includes a plurality of subbands. When only a single rankis determined for the system bandwidth, efficiency can be lowered if thesystem bandwidth is large. For example, only a single rank can beselected in a system having a bandwidth of 5 MHz or smaller. In a systemhaving a bandwidth of 5 MHz or larger, i.e., 10 MHz, 15 MHz, 20 MHz, orthe like, the system bandwidth can be divided into a plurality ofprimary bands, and a single rank can be determined for each of theprimary bands.

The number of primary bands can be differed depending on the size of thesystem bandwidth. The size of primary bands can be uniform or can bedifferent with each other.

A UE selects a single rank for each of the primary bands and feeds backa CQI corresponding to the selected single rank. A BS transmits resourceallocation information to the UE. The BS can transmit the resourceallocation information through only one L1/L2 control signal.

In addition, the BS can allocate subbands having the same single rank tothe UE. For example, it is assumed that a primary band #1 is determinedas rank 1, a primary band #2 is determined as rank 1 and a primary band#3 is determined as rank 2. The BS can allocate primary bands #1 and #2having the same rank to the UE.

FIG. 6 is a flowchart illustrating a method for transmitting dataaccording to an embodiment of the present invention.

Referring to FIG. 6, a UE determines a single rank for overall subbandsand transmits channel information including the single rank and a CQIfor each subband of the single rank S310. A CQI is reported for eachsubband and only the single rank is reported for overall subbands.Therefore, signaling overhead due to reporting channel information canbe reduced.

A BS transmits radio resource information allocated to the UE S320. Theradio resource information can be transmitted through a downlink controlchannel, such as a L1/L2 control channel, dedicated control channel orthe like. The radio resource information includes a rank used fordownlink data and information on the allocated subband. The BS candetermine the rank to be used for transmitting the downlink data usingthe single rank and inform the UE of the determined rank through thedownlink control channel. Alternatively, the BS can determine the rankby overriding the single rank.

The BS transmits downlink data to the UE through the allocated subbandS330.

A user equipment selects a single rank for overall subbands based on aspecific criterion and reports only the single rank. Radio resourcesrequired for reporting channel information can be reduced and signalingoverheads can be minimized.

The steps of a method described in connection with the embodimentsdisclosed herein may be implemented by hardware, software or acombination thereof. The hardware may be implemented by an applicationspecific integrated circuit (ASIC) that is designed to perform the abovefunction, a digital signal processing (DSP), a programmable logic device(PLD), a field programmable gate array (FPGA), a processor, acontroller, a microprocessor, the other electronic unit, or acombination thereof. A module for performing the above function mayimplement the software. The software may be stored in a memory unit andexecuted by a processor. The memory unit or the processor may employ avariety of means that is well known to those skilled in the art.

As the present invention may be embodied in several forms withoutdeparting from the spirit or essential characteristics thereof, itshould also be understood that the above-described embodiments are notlimited by any of the details of the foregoing description, unlessotherwise specified, but rather should be construed broadly within itsspirit and scope as defined in the appended claims. Therefore, allchanges and modifications that fall within the metes and bounds of theclaims, or equivalence of such metes and bounds are intended to beembraced by the appended claims.

1. A method of reporting downlink channel information to a base stationin a multiple antenna system, the method comprising: reporting a singlerank for overall subband, the overall subband comprising a plurality ofsubbands; and reporting a CQI for the single rank for at least onesubband.
 2. The method of claim 1, wherein after selecting at least onesubband among the plurality of subbands, the CQI for the selectedsubband is reported.
 3. The method of claim 2, further comprising:reporting an average CQI for the other subbands.
 4. The method of claim1, further comprising: receiving a rank used for downlink data from thebase station through a downlink control channel.
 5. A method ofreporting downlink channel information to a base station in a multipleantenna system, the method comprising: selecting a single rank foroverall subband, the overall subband comprising a plurality of subbands;reporting the single rank; and reporting a CQI for the single rank foreach subband.
 6. The method of claim 5, wherein selecting the singlerank comprises calculating an signal-to-interference plus noise ratio(SINR) for each subband; calculating metrics for every ranks using theSINR; and selecting the single rank to which a subband having thelargest metric belongs.
 7. The method of claim 6, wherein a metric is athroughput of each rank.
 8. The method of claim 6, wherein a metric is acapacity of each rank.
 9. A method for transmitting downlink data in amultiple antenna system, the method comprising: receiving a single rankfor overall subband; receiving a CQI for the single rank; transmitting arank determined using the single rank through a downlink controlchannel; allocating at least one subband using the rank and the CQI; andtransmitting the downlink data through the allocated subband.
 10. Themethod of claim 9, wherein the subband includes twelve subcarriers.